1. Field of the Invention
The present invention relates to a microwave circuit, and a method for coupling distributed constant lines with each other in case of integrating plural chips having provided therein microwave circuits, particularly, distributed constant circuits.
2. Description of the Related Art
A microwave circuit is a so-called distributed constant circuit in which distributed constant lines are used for guiding a microwave signal. This specification assumes as a distributed constant circuit a circuit composed of a conductor line, a dielectric substrate, and a grounding conductor. In this specification, a distributed constant circuit is sometimes used in a sense of a microwave circuit. Up to now, in case of integrating plural distributed lines to make a microwave circuit, the respective distributed constant lines have been coupled with each other by bonding wires, ribbons, and the like to the lines (reference: Microwave Semiconductor Circuits, FIG. 6.13, pp.139, issued by The Nikkan Kogyo Shimbun Ltd., in 1993).
FIG. 18 is a perspective view showing an example of a conventional construction disclosed in this reference, and in this perspective view there is shown a conventional coupling between a first distributed constant line 70 and a second distributed line 72. The first distributed constant line 70 is composed of a first grounding conductor 12, a first dielectric substrate 14, and a first conductor line 46 which are stacked one after another. The second distributed constant line 72 is composed of a second grounding conductor 13, a second dielectric substrate 16, and a second conductor line 48 which are stacked one after another. In FIG. 18, to simplify the figure, only distributed constant lines are shown and the other circuit elements usually provided are omitted even if they are provided on these dielectric substrates 14 and 16. In the construction shown in FIG. 18, therefore, the coupling between the first distributed constant line 70 and the second distributed constant line 72 has used a mechanical bridging member, for example, a wire 26 for coupling the first conductor line 46 and the second conductor line 48 with each other and an underlayer metal plate 10 for coupling the first grounding conductor 12 and the second grounding conductor 13 with each other.
As described with reference to FIG. 18, however, when the first and second distributed constant lines 70 and 72 are provided on the underlayer metal plate (grounding metal plate) 10, a gap of at least 0.1 to 0.2 mm or so comes to be generated between the distributed constant lines 70 and 72 (between the dielectric substrates 14 and 16), depending upon the dimensional accuracy of these distributed constant lines (mainly the dimensional accuracy of the dielectric substrates) or their assembling accuracy (accuracy related to positioning or processing). Accordingly, for example, in case of coupling the first and second conductor lines 46 and 48 with each other by bonding the wire 26, respectively, with them, an inductance of at least 0.1 to 0.2 nH or so results in being inserted in series between the first and second conductor lines 46 and 48.
As a result, an impedance mismatching between the first and second conductor lines 46 and 48 (namely, an impedance mismatching between the microwave circuits which are respectively composed of the first and second distributed constant lines 70 and 72) comes to be generated, and there has been a problem that characteristics (signal propagation characteristics such as gain, voltage standing wave ratio (VSWR), and the like) of the microwave circuit composed of both the first and second distributed constant lines 70 and 72, are deteriorated. And the higher the operating frequency is, the poorer the obtained characteristics have been.
Accordingly, it is an object of the present invention is to provide a method for coupling distributed constant lines with each other without deteriorating their signal propagation characteristics.
Another object is to provide a microwave circuit in which distributed constant lines are coupled with each other while keeping their good signal propagation characteristics.